With the development of display technology, liquid crystal displays have been widely used in a variety of display areas, including home, public places, offices, personal electronics products and etc. A liquid crystal display panel mainly comprises a liquid crystal cell formed by an array substrate and a color filter substrate, a polarizing plate and a backlight module and etc. A great number of Thin Film Transistors (TFTs) are formed at intersections of gate lines and data lines, wherein a gate line controls a TFT to be turned on or turned off. When a TFT is turned on, a pixel electrode is charged or discharged through a data line, and a magnitude of a voltage applied to the liquid crystal molecules is controlled, so that the light through the liquid crystal molecules can display different grayscales. A circuit for driving gates is called a gate driving circuit, which sequentially outputs a scanning signal to the gate lines, wherein the scan signal is normally generated by a shift register. Because of different requirements on a display device, sometimes it is necessary for the display device to achieve a reverse display effect, which requires a shift register as a gate driving circuit having a bidirectional scanning function.
A common structure of an existing shift register unit for bidirectional scanning is illustrated in FIG. 1. The structure mainly comprises: a control unit 11 comprising inside a specific design for a forward scanning and a reverse scanning of the shift register unit, and a output unit 12 mainly comprising a pull-up transistor Tpu and a pull-down transistor Tpd, wherein gates of the pull-up transistor Tpu and the pull-down transistor Tpd are connected to control nodes PU and PD of the control unit 11 respectively, a drain of the pull-up transistor Tpu receives a clock signal CLK, and a source of the pull-down transistor Tpd is connected to a low level voltage terminal VSS. The control unit 11 is connected to an output terminal Vouti-1 of a previous stage shift register unit and receives the clock signal CLK. When receiving an output signal from the output terminal Vouti-1 of the previous stage shift register unit, the control unit 11 allows the node PU to be charged so as to turn on the pull-up transistor Tpu and simultaneously allows the node PD to be discharged so as to turn off the pull-down transistor Tpd; when the clock signal CLK is at a high level, the clock signal CLK at the high level is output from an output terminal Vouti via the transistor Tpu; in addition, when the clock signal CLK is at a low level, the control unit 11 allows the node PU to be discharged and the node PD to be charged simultaneously, and a low level of VSS is output from the output terminal Vouti via the transistor Tpd.
Such a shift register unit for bidirectional scanning has the following defect: the threshold voltage of the pull-down transistor Tpd drifts due to a DC voltage bias, which has an adverse influence on stability of the shift register unit seriously and affects quality of a product including the same.